Arsenic Mobilization from Silicic Volcanic Rocks in the Southern Willamette Valley

Volcanic tuffs and tuffaceous sediments are frequently associated with elevated As groundwater concentrations even though their bulk As contents (∼ 5 mg kg-1; Savoie, 2013) are only marginally greater than the average crustal abundance of 4.8 ?g g-1 (Rudnick & Gao, 2003). Thus, As mobilization must be facilitated by conditions particular to these rocks. Alkaline desorption, anionic competition, reactive glass dissolution, and reductive dissolution of iron oxides are proposed processes of As release from volcanic rocks. Geogenic As contamination of groundwater in the southern Willamette Valley in western Oregon has been well-documented since the early 1960s, and previous studies have identified the Little Butte Volcanics Series and Fisher and Eugene Formations as the source of As contamination.;This study examines 19 samples from 10 units of ash flow tuffs and tuffaceous sediments within the Fisher Formation and Little Butte Volcanics Series, representing a range of weathering and devitrification, to determine conditions of mobilization and mineralogical constraints that control As release into solution. Leachate studies were conducted over a range of pH from 7 to 11, phosphate concentrations from 10 muM to 100 mM, and in time series from 4 to 196 hours. Results demonstrate that silicic volcanic tuffs are capable of mobilizing As in concentrations above regulatory limits at pH conditions produced naturally by the tuffs (pH 8-9) or with moderate concentrations of P (10-100 muM). Alteration products, e.g. zeolites and clays, appear to be the primary host phases for mobile As. Samples that do not contain these alteration products tend to produce concentrations of As well below regulatory limits and often below the instrument detection limits of this study. The type of alteration may influence As mobilization: tuffs containing more clays tend to mobilize As through surficial desorption, and tuffs containing more zeolites tend to mobilize As by dissolution or formation of colloids. Additionally, one volcaniclastic sample demonstrates that extremely elevated concentrations of As, up to 1000 mug/L are possible as a result of oxidative dissolution of As-bearing sulfide phases.